A typical thermal head is provided with a plurality of exothermic bodies which generate heat when electricity is applied thereto. The thermal energy generated by the exothermic bodies is supplied to a recording medium in order to perform a recording operation. Conventionally, a thermal head is provided with a head substrate and a printed circuit board, which are attached on an aluminum heat sink. The head substrate includes the plurality of exothermic bodies, a common electrode connected with first ends of the exothermic bodies, and a plurality of independent electrodes connected with corresponding second ends of the exothermic bodies. On the other hand, the printed circuit board includes a plurality of driving elements for controlling the electricity supplied to the exothermic bodies via the independent electrodes of the head substrate. The head substrate and the printed circuit board are disposed on the aluminum heat sink in a manner such that an end surface of the head substrate and an end surface of the printed circuit board adjoin each other. The independent electrodes of the head substrate and the driving elements of the printed circuit board are correspondingly wire-connected with each other across a bordering section between the head substrate and the printed circuit board so as to form a wire-connection unit. Such a wire-connection unit is sealed with a sealing-resin material, such as epoxy resin, whose viscosity decreases when heat is applied thereto. The sealing-resin material covers the wire-connection unit without leaving any empty spaces.
In the thermal head having the structure described above, it is actually difficult to make the end surface of the printed circuit board adjoining the head substrate exactly flat, and due to this reason, a small gap is formed between the head substrate and the printed circuit board. When a heated sealing-resin material in a low viscosity state enters the gap between the head substrate and the printed circuit board, the sealing-resin material traps air inside the gap, causing the air to expand. As a result, air bubbles are formed in the sealing-resin material. The formation of such air bubbles can lead to problems such as uneven surfaces or air holes formed in the hardened sealing-resin material. To prevent such problems, Japanese Unexamined Patent Application Publication No. 7-323592, for example, discloses a structure in which a section of the heat sink below the sealing portion in the gap between the head substrate and the printed circuit board is provided with a groove through which the air can be released outward.
However, according to such a heat sink provided with a groove, it has been discovered that the heated sealing-resin material in a low viscosity state moves further past the gap between the head substrate and the printed circuit board to enter the groove in the heat sink, and as a result, the sealing-resin material adheres to an elastic adhesive provided for an attachment between the head substrate and the heat sink. If the sealing-resin material adheres to the elastic adhesive, the head substrate and the heat sink become tightly combined with each other. This increases the rigidity between the head substrate and the heat sink, or in other words, reduces the elasticity between the two. In this case, even when the sealing-resin material hardens while the heat sink is thermally expanded, the elastic adhesive cannot absorb such thermal expansion of the heat sink. This forces the top and bottom surfaces of the head substrate to warp significantly. In order to increase the yield, the head substrate is kept flat as much as possible.